This invention relates to a read head for providing an electrical pulse in response to a switch in state of the magnetic wire known as the Wiegand wire.
The Wiegand wire is a ferro magnetic wire having core and shell portions with divergent magnetic properties. These wires have come to be known in the art as Wiegand wires. The currently preferred type of Wiegand wire is disclosed in U.S. Pat. No. 4,247,601 issued on Jan. 27, 1981. Read heads which are effective to provide an output pulse from a switch in state of the Wiegand wire are described in U.S. Pat. No. 4,263,523 issued Apr. 21, 1981 and U.S. Pat. No. 4,593,209 issued June 3, 1986. A module employing the Wiegand wire that is effective to generate a pulse in response to a change in magnetic field is described in U.S. Pat. No. 4,484,090 issued Nov. 20, 1984.
Read heads for use with a Wiegand wire are currently employed in various access systems. Codes are incorporated in access cards and keys where the Wiegand wire is employed in the card or key to provide the encoding. One technique of positioning these wires in the access card and for reading the wires as the wires are passed over the face of a read head is described in said U.S. Pat. No. 4,593,209.
The manner in which the Wiegand wires are encoded on a code strip carried in an access card is described in connection with the discussion of FIGS. 5 and 6 in said U.S. Pat. No. 4,593,209. As shown therein, the "zero" bit wires are all parallel to one another in a single column like the rungs of a ladder. The "one" bit wires are also parallel to one another in a single column. However, the center lines of the two columns are spaced from one another. Thus, the zero bits are read by one portion of the read head and the one bits are read by another portion of the read head.
The read head designs previously known impose a predetermined limitation on the density with which these bits may be packed. If the bits are closer together than a certain amount a phenomenon known as bit switching occurs. What occurs is that when a series of one bits occur in sequence, they tend to magnetically load or bias the read head in such a fashion that the next successive zero bit tends to switch state prematurely inducing a zero bit pulse output prior to the switch in state of the last of the succession of one bits. The complementary bit switching occurs when a series of zero bits occurs in sequence. This bit switching phenomenon places a limitation on how densely packed the bits may be. This limitation is a function of read head design.
It is an object of this invention to provide a read head design which will permit a more densely packed bit arrangement than has been possible before.
The advantage of a more densely packed set of bits is that it permits more extensive encoding on a predetermined size card and also permits the creation of a smaller access key for a given amount of coding.
A Wiegand wire switches at a threshold value of a changing magnetic field. The more precisely the position can be determined where the switching occurs, the smaller can be the design of the read head and the more densely can the bits be packed in the access card or key.
Accordingly, it is a further purpose of this invention to provide a read head which provides a more precisely determined positional point of switching for the Wiegand wires and, in particular, to provide a steep gradient for the magnetic field in the zone of switching.
In general, and in particular with widely used access cards and keys, it is desirable to provide an access system which is as small as possible and as inexpensive as possible. Accordingly, it is a further object of this invention to provide the above objects in a context which minimizes size and cost.